1. Field of the Invention
The present invention relates to an opto-mechanical sensor for providing load and/or pressure distribution data, and especially to a pressure sensor providing high spatial resolution over a large area on a surface exposed to constant or varying forces. The system can provide large quantities of information in a visual and digital format using simple and easily implemented techniques. The sensor may have a large area, of several square meters, or a small area of less than one square centimeter. The pressures to be measured may also vary greatly.
2. Prior Art
Loads and pressure distributions on surfaces have traditionally been measured using mechanical load cells for the force on the whole surface and pressure sensors incorporated into the surface for local force and pressure measurement. The load cells are made of a material, usually metal, that deforms under load and the strain is monitored using an attached strain gauge. The pressure sensors are generally mechanical in design.
One type of pressure sensor is the piston/diaphragm design where load on the pressure head is transmitted through the piston to the diaphragm causing a deflection and a consequent change in output from a strain sensor on the diaphragm. Piezoelectric films are also available for measuring dynamic load and pressure. These existing technologies have drawbacks.
Firstly, the piston/diaphragm sensors require fairly extensive modification of the surface for installation and are limited in the number that can be installed so that large areas cannot be instrumented nor can the spatial resolution be very great because of structural modifications to the surface. Every sensor must have appropriate electrical connections which further limit the number and extent of the sensors on a surface.
Piezoelectric film has the advantage over piston sensors that the surface does not require much alteration and the spatial resolution can be high for the sensor spacing. The sensors can only be used for dynamic load situations however and there is still the issue of wiring being required for each of the sensing elements in a sensor array. They are impractical for coverage of large areas at high spatial resolution because of the demands on the data acquisition system. For example a one metre by one metre array of sensors with one centimetre spatial separation implies that there are 10,000 individual sensors from which to acquire data.
Surface-area transducers using electrical components such as capacitors to measure pressure distribution are also known, for example as described in U.S. Pat. No. 4,644,801 to Kustanovich, issued Feb. 24, 1987. Such transducers also require rather complex data acquisition systems.
U.S. Pat. No. 4,599,908 to Sheridan et al., issued Jul. 15, 1986, describes a system in which a pressure deformable body has an array of holes which are each aligned with a hole in a supporting base or platen, and each of the base or platen holes is fitted with the end of an optical fiber. The upper surface of the deformable body is covered by a load receiving flexible sheet, and when the flexible sheet is subjected to loads it causes the deformable body to bulge into its holes to reduce the hole diameter. The optical fibers are connected to a receiver viewed by a video camera, and the optical fibers transmit signals to the camera which indicate the reduction in hole diameter caused by the pressure. The Sheridan et al. system has drawbacks similar to those systems having a large number of load sensors, in that the fibers have to be attached individually to the holes, and a one metre by one metre array at one centimetre spacing would require 10,000 fibers. With less fibers, the resolution will be low. The need for a fiber connection corresponding to each hole also limits the nature of the deformable body; it needs an array of holes matching those of the base, which involves substantial expense, and cannot be formed as a body having a series of slits, as may be desirable for economy.
Other pressure distribution sensors use optical fibers in the plane of the sensors and which are sensitive to bending of the fibers or to contacts made between fibers when pressed together under pressure; an example is U.S. Pat. No. 4,901,584, issued to Brunner et al. on Feb. 20, 1990.
A pressure sensor is also known from U.S. Pat. No. 3,987,668, issued to Popenoe on Oct. 26, 1976. This uses a flexible light transmitting member which, under pressure, is pushed into contact with a light absorbing member; the area of contact between the light transmitting member and the light absorbing member is recognized by the frustration of internal reflection which occurs in this area. However, this sensor is only suitable for indicating overall pressure, and it is not suitable for showing pressure distribution since any area subjected to pressure affects neighbouring areas.